fix static linking dependency bloat with cancellation

previously, pthread_cleanup_push/pop were pulling in all of
pthread_create due to dependency on the __pthread_unwind_next
function. this was not needed, as cancellation cleanup handlers can
never be called unless pthread_exit or pthread_cancel is reachable.

src/thread/cancellation.c

@@ -3,10 +3,22 @@
 #ifdef __pthread_register_cancel
 #undef __pthread_register_cancel
 #undef __pthread_unregister_cancel
+#undef __pthread_unwind_next
 #define __pthread_register_cancel __pthread_register_cancel_3
 #define __pthread_unregister_cancel __pthread_unregister_cancel_3
+#define __pthread_unwind_next __pthread_unwind_next_3
 #endif
 
+static void dummy(struct __ptcb *cb)
+{
+}
+weak_alias(dummy, __pthread_do_unwind);
+
+void __pthread_unwind_next(struct __ptcb *cb)
+{
+	__pthread_do_unwind(cb);
+}
+
 void __pthread_register_cancel(struct __ptcb *cb)
 {
 	struct pthread *self = pthread_self();

src/thread/i386/cancellation2.s

@@ -14,3 +14,11 @@ __pthread_unregister_cancel:
 	call __pthread_unregister_cancel_3
 	popl %eax
 	ret
+
+.global __pthread_unwind_next
+.type   __pthread_unwind_next,@function
+__pthread_unwind_next:
+	pushl %eax
+	call __pthread_unwind_next_3
+	popl %eax
+	ret

src/thread/i386/cancellation3.s

-.text
-.global __pthread_unwind_next
-.type   __pthread_unwind_next,@function
-__pthread_unwind_next:
-	pushl %eax
-	call __pthread_unwind_next_3
-	popl %eax
-	ret

src/thread/pthread_create.c

@@ -8,12 +8,7 @@ weak_alias(dummy_0, __synccall_lock);
 weak_alias(dummy_0, __synccall_unlock);
 weak_alias(dummy_0, __pthread_tsd_run_dtors);
 
-#ifdef __pthread_unwind_next
-#undef __pthread_unwind_next
-#define __pthread_unwind_next __pthread_unwind_next_3
-#endif
-
-void __pthread_unwind_next(struct __ptcb *cb)
+void __pthread_do_unwind(struct __ptcb *cb)
 {
 	pthread_t self = pthread_self();
 	int n;